1. Field of Invention
This invention pertains to the multi-stage catalytic hydrogenation conversion of monosaccharides, such as glucose, to produce alditol products, such as high-purity sorbitol. It pertains more particularly to a continuous process for catalytic hydrogenation of such monosaccharides using a bace-mixed type reaction zone as first stage and at least one fixed-catalyst-bed type reactor connected in series, and all operated to control pH of the reaction zone liquid and maintain catalyst activity while producing a high-purity sorbitol product.
2. Description of Prior Art
Sorbitol can be produced commercially from glucose through either electrolytic reduction, enzymatic, or catalytic hydrogenation processes, but for economic reasons the catalytic hydrogenation processes for making sorbitol have largely replaced the other two processes. A batch autoclave reaction process using a Raney nickel powder catalyst is presently the major glucose hydrogenation process used in industry. However, batch processes have the disadvantage that new catalyst must be made in situ for each batch of feed. Although such catalyst can be filtered and reused, about 25% is lost and must be replaced with new catalyst. Another disadvantage of batch process plants is that their annual capacity relative to reaction volume is very small, which requires large, expensive reactors. Also, their requirements of steam, power and labor are relatively high. To overcome these batch process handicaps, a continuous hydrogenation process using a suspended Raney nickel catalyst powder and two stirred tank reactors in series was developed and is used by some companies, as described by Haideggar et al in Industrial & Engineering Chemistry Vol. 7, No. 1, January 1968, pp. 107-110. However, a remaining disadvantage is the need to filter the catalyst from the product liquid in order to recycle the catalyst, and doing this without reducing the activity of the recycled catalyst by its exposure to air (oxygen).
It has been further reported by the Haidegger article that VEB Deutsches Hydrierwork uses a continuous catalytic fixed-bed process for converting glucose to sorbitol, using a supported mixed copper/nickel catalyst. However, this process requires a relatively high hydrogen pressure of almost 3000 psig, and low liquid hourly space velocity. It was further reported by Haidegger that local overheating of the catalyst surface due to the heat of the hydrogenation reaction led to isomerization, cracking, and carmel formation, so that the product sorbitol contained a significant amount of mannitol. Thus, flow conditions in such fixed-catalyst-bed processes were regarded as less desirable than in continuous reactors using suspended catalysts.
Other processes for the batch-type catalytic hydrogenation of sugars to produce sorbitol and similar products are described in U.S. Pat. Nos. 1,963,999 and
1,990,245. U.S. Pat. No. 2,650,941 to Koome discloses a continuous process for catalytic conversion of carbohydrates in aqueous solutions to produce polyhydric alcohols, using one or more fixed-bed reactors connected in series. Useful reaction conditions are within the range of 100.degree.-165.degree. C. and 45-200 atmospheres hydrogen pressure. However, liquid space velocities were low, being only about 0.15 liter of 40% glucose per liter of catalyst in both reactors to achieve a maximum glucose conversion of 98.8%, while catalyst life was not disclosed.
U.S. Pat. No. 3,329,729 to Brandner discloses a multi-stage batch-type catalytic hydrogenation process for converting a mixture of glucose and fructose to produce mannitol and sorbitol. The feed contains 20-80% by weight sugars, and the catalyst contains about 20% nickel. Useful reaction conditions are in the range of 50.degree.-80.degree. C. temperature and hydrogen pressure of 500-3000 psig. U.S. Pat. No. 3,538,019 to Capik discloses a catalyst composed of nickel and nickel phosphate on an inert carrier, having total nickel of 12-45 W %. Such catalysts are useful for producing polyhydric alcohols from carbohydrates, and are used as suspensions with the carbohydrate feed. Useful reaction conditions are 120.degree.-210.degree. C. temperature and 25-200 atmospheres pressure.
There remains a commercial need for an economical continuous process to produce high-purity, USP-grade sorbitol solution from glucose, which specifies a maximum content of reducing sugars, including glucose of only 0.21 W %. An earlier process developed by applicants used a continuous fixed-bed single-stage catalytic hydrogenation process for converting glucose to sorbitol. By using a preferred high nickel-containing catalyst and under preferred operating conditions, 90 to 99% of glucose in the feed can be converted to sorbitol, and the catalyst activity can be maintained for up to about 100 hours before regeneration is required. However, the catalyst life for this process needs to be increased appreciably to reduce the cost of catalyst regeneration or replacement. Also, because a USP-grade sorbitol solution must contain only a minimal amount of reducing sugars, this requires at least 99 W % conversion of a feedstock containing 40 W % glucose to sorbitol product. It was unexpectedly observed that acidity develops in the reaction solution and is a major cause for deactivation of the catalyst. Gluconic acid is generated in the process as a side product and leaches nickel metal from the catalyst, thus causing deactivation of the catalyst and contamination of the product with nickel. Such deactivation does not occur with a ruthenium catalyst as described in U.S. Pat. No. 2,868,847 to Boyers; however, the ruthenium catalyst tested had a much lower activity than nickel catalyst.
In order to achieve the goals of producing USP-grade sorbitol solution and also substantially extending catalyst useful life, an improved multi-stage process sequence was conceived and successfully tested. Results showed the effectiveness of this new process for achieving the requirements of producing a USP-grade sorbitol solution product and maintaining long life for the catalyst.